Bright nickel electroplating baths are used in the automotive, electrical, appliance, hardware and various other industries. One of the most commonly known and used nickel electroplating baths is the Watts bath. A typical Watts bath includes nickel sulfate, nickel chloride and boric acid. The Watts bath typically operates at a pH range of 2-5.2, a plating temperature range of 30−70° C. and a current density range of 1-6 amperes/dm2. Nickel sulfate is included in the baths in comparatively large amounts to provide the desired nickel ion concentration. Nickel chloride improves anode corrosion and increases conductivity. Boric acid is used as a weak buffer to maintain the pH of the bath. In order to achieve bright and lustrous deposits, organic and inorganic brightening agents are often added to the baths. Examples of typical organic brighteners are sodium saccharinate, naphthalene trisulfonate, sodium allyl sulfonate, coumarin, propargyl alcohol and diethyl propargyldiol.
Although many conventional additives for nickel electroplating baths have sufficed to provide semi-bright to bright nickel deposits as well as uniformity of appearance and plating speeds, in general, multiple additives are included to achieve the desired nickel plating performance. In some nickel electroplating compositions as many as six additives are included to achieve the desired nickel plating performance and deposit. A disadvantage of such nickel electroplating baths is the difficulty in controlling the bath performance and deposit appearance. To achieve the desired bath performance and deposit appearance the additives must be in proper balance, otherwise an inferior and unacceptable nickel deposit is obtained and plating performance is inefficient. Workers using the bath necessarily have to monitor the concentrations of bath additives and the greater number the additives in the bath the more difficult and time consuming it is to monitor the bath. In addition to the large number of additives, the presence of many different types of additives makes quantitative monitoring of each additive of the bath impractical and unreliable. During plating many of the bath additives breakdown into compounds which can compromise nickel plating. Some additives are included in the baths at concentrations as high as 5 g/L. The higher the concentration of the additives the greater the breakdown products. The breakdown products must be removed at some point during the plating process and the nickel baths must be replenished with new additives to compensate for the additives which have broken down to maintain plating performance and deposit quality. Additive replenishment should be substantially accurate. Another problem associated with high concentrations of additives in nickel plating baths is that additives can co-deposit with the nickel which negatively impacts the properties of the deposit causing embrittlement and increased internal stress. Ductility of the nickel deposit is also compromised. Sulfur containing additives are particularly pernicious in their effects on ductility.
An example of a conventional non-sulfur containing nickel bath additive which has had mixed performance is coumarin. Coumarin has been included in nickel plating baths to provide a high-leveling, ductile, semi-bright and sulfur-free nickel deposits from a Watts bath. Leveling refers to the ability of the nickel deposit to fill in and smooth out surface defects such as scratches and polish lines. An example of a typical nickel plating bath with coumarin contains about 150-200 mg/L coumarin and about 30 mg/L formaldehyde. A high concentration of coumarin in the bath provides very good leveling performance; however, such performance is short-lived. Such high coumarin concentrations result in a high rate of detrimental breakdown products. The breakdown products are undesirable because they can cause non-uniform, dull gray areas in the deposit that are not easily brightened by subsequent bright nickel deposits. They can reduce the leveling performance of the nickel bath as well as reduce other beneficial physical properties of the nickel deposit. To address the problem workers in the industry have proposed to reduce the coumarin concentrations and add formaldehyde and chloral hydrate; however, use of such additives in moderate concentrations not only increases tensile stress of the nickel deposits but also compromise leveling performance of the baths. Further, many government regulations, such as REACh, consider formaldehyde, as well as coumarin compounds harmful to the environment. Therefore, use of such compounds is discouraged in the plating industry.
It is important to provide highly leveled bright nickel deposits without sacrificing deposit ductility and internal stress. The internal stress of the plated nickel deposit can be compressive stress or tensile stress. Compressive stress is where the deposit expands to relieve the stress. In contrast, tensile stress is where the deposit contracts. Highly compressed deposits can result in blisters, warping or cause the deposit to separate from the substrate, while deposits with high tensile stress can also cause warping in addition to cracking and reduction in fatigue strength.
As briefly mentioned above, nickel electroplating baths are used in a variety of industries. Nickel electroplating baths are typically used in electroplating nickel layers on electrical connectors and leadframes. Such articles have irregular shapes and are composed of metal such as copper and copper alloys with relatively rough surfaces. Therefore, during nickel electroplating, the current density is non-uniform across the articles often resulting in nickel deposits which are unacceptably non-uniform in thickness and appearance across the articles.
Accordingly, there is a need for nickel electroplating compositions and methods to provide bright and uniform nickel deposits, even across a wide current density range, good ductility and which have a reduced number of additives.